WebView: add @Nullable and @NonNull annotations

This adds @Nullable and @NonNull annotations where the docs previously
stated this. This change should be safe, since we already disallowed
null values for the @NonNull parameters via documentation.

I verified that documentation changes for APIs which previously did not
mention null-ness in @param or @return Javadocs (if the API already
mentioned it in these spots, then DroidDoc prefers the hand-written
description).

This also fixes various lint errors.

Bug: 65465498
Test: make update-api and manually verify docs
Change-Id: I4751508d0e72be8ddfc3d6b601db8c307c9df60e